An artifact built by Maven can be declared as a Maven plugin by specifying the packaging as maven-plugin in the pom.xml.

<packaging>maven-plugin</packaging>

You need to declare a dependency on the plugin API and annotations.

<dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-plugin-api</artifactId>
    <version>3.3.9</version>
    <scope>provided</scope>
</dependency>
<dependency>
    <groupId>org.apache.maven.plugin-tools</groupId>
    <artifactId>maven-plugin-annotations</artifactId>
    <version>3.5</version>
    <scope>provided</scope>
</dependency>

You need to add a plugin to generate the metadata.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-plugin-plugin</artifactId>
    <version>3.5</version>
</plugin>

Goals are implemented by creating a MOJO. This is a class file annotated with annotations from maven-plugin-annotations.

@Mojo(name = "hello")
public final class HelloWorldMojo extends AbstractMojo {

    public void execute() throws MojoExecutionException, MojoFailureException {
         getLog().info("Hello world");
    }
}

Plugins can be configured by annotating fields with @Parameter. The MOJO is then injected with the configuration.

@Mojo(name = "greet")
public final class GreetMojo extends AbstractMojo {

    @Parameter(required = true)
    public String name;

    public void execute() throws MojoExecutionException, MojoFailureException {
        getLog().info("Hello " + name);
    }
}

The name parameter can be configured in the POM:

<plugin>
    <groupId>com.mattunderscore</groupId>
    <artifactId>hello-world-plugin</artifactId>
    <version>1.0-SNAPSHOT</version>
    <configuration>
        <name>Matt</name>
    </configuration>
</plugin>

If the greet goal is run as a standalone goal the name parameter can be defined as property on the command line:

mvn <plugin name>:greet -Dname=Geri

The plugin can, among others, access information about the current Maven project being built.

@Mojo(name = "project")
public final class ProjectNameMojo extends AbstractMojo {

    @Parameter(defaultValue = "${project}", readonly = true, required = true)
    private MavenProject project;

    public void execute() throws MojoExecutionException, MojoFailureException {
        getLog().info("Hello, this is " + project.getName());
    }
}

The above example would print in the console the name of the Maven project it is run on, which is specified in the <project>/<name> element of its POM.

The MavenProject class used in the plugin requires a dependency of maven-core with a (default) compile scope in the plugin's POM:

<dependency>
    <groupId>org.apache.maven</groupId>
    <artifactId>maven-core</artifactId>
    <version>3.3.9</version>
</dependency>

Furthermore, using annotations requires the following dependency in the plugin's POM:

<dependency>
  <groupId>org.apache.maven.plugin-tools</groupId>
  <artifactId>maven-plugin-annotations</artifactId>
  <version>3.5</version>
  <scope>provided</scope> <!-- annotations are needed only to build the plugin -->
</dependency>

@Mojo(name = "hi", defaultPhase = LifecyclePhase.COMPILE)

@Parameter(defaultValue = "${project.build.directory}")
private File buildDirectory;